With explosive increase of a demand for a broadband multimedia communication service such as internet, it is necessary to develop a larger capacity and higher function optical fiber communication system. The number of optical communication modules used in such a large-scaled system increases due to a large scale tendency of the system. Costs and mounting loads in the overall system including the size of the optical communication module are not ignorable any more. Therefore, miniaturization, function integration and low costs of the optical communication module become very important tasks.
Particularly, as a method for implementing an optical subscriber system such as a Fiber To The Home (FTTH), a single core bidirectional light transmission/reception module where a light signal function of sending a signal and a light reception function are integrated on one optical platform is expected as an optical integration technology that can be possibly brought into practical use in terms of miniaturization and low cost.
FIG. 8 shows a conceptual view of the single core bidirectional light transmission/reception module. Optical fiber 82 is arranged on silicon platform 81, and semiconductor laser 83 for outputting a signal light having a wavelength of e.g. 1.3 μm is fixed to a front end of optical fiber 82. Wavelength Division Multiplex (WDM) filter 84 is installed along optical fiber 82, and light-receiving device 85 for a reception signal is fixed just above WDM filter 84. An output of semiconductor laser 83 becomes light λ2 transmitted to the outside via optical fiber 82. Reception light λ1 incident from the outside via optical fiber 82 is normally signal light having a longer wavelength than that of transmission light λ2, e.g. a wavelength of 1.5 μm, reflected by WDM filter 84, incident on light-receiving device 85, and detected by a light-receiving portion.
In the single core bidirectional light transmission/reception module with the above construction, since transmission and reception devices are accommodated in one package adjacently to each other for package miniaturization, various scattered light is generated in the module by transmission light λ2. Therefore, light other than light to be received, such as the scattered light, intrudes into light-receiving device 85, to thereby cause optical crosstalk. It is thus necessary to restrict the optical crosstalk. Accordingly, a countermeasure is conducted, arranging a high sensitivity filter at a front end of light-receiving device 85 to selectively transmit light λ1 to be received.
However, in the related art, when the WDM filter is provided with high sensitivity, the cost increases, and when a band-pass filter is used in addition to the WDM filter, the number of components increases. Such increase of the number of the components undesirably results in increase of the cost and size of the module.
The light-receiving devices are classified into a rear face incident type light-receiving device where reception light is incident from a semiconductor substrate side to a light-receiving layer stacked on the semiconductor substrate, and an end face incident type light-receiving device where an electrode is arranged at a portion of a mesa-structure semiconductor waveguide as a light-receiving portion to receive light incident from a mesa end face. Also, known is a surface incident type light-receiving device where a multiple reflection layer is arranged on a semiconductor substrate, and a light-receiving layer is formed thereon, such that incident light from the light-receiving layer side is reflected by the multiple reflection layer and received by the light-receiving layer.
Known rear face incident type light-receiving devices are a planar type light-receiving device and a mesa type light-receiving device. In addition, there are a structure where electrodes forming a pair are arranged on a light incident side and an opposite side, respectively, and a structure where electrodes are arranged merely on an opposite side to a light incident side. A known mesa type light-receiving device is a flip chip mountable device where a second electrode is extended or formed on a mesa specially arranged with the same height as that of a first electrode arranged on a light-receiving portion mesa structure (e.g. refer to Drawings 3 and 4 of Patent Document 1).
In order to reduce optical crosstalk without increasing the number of components, suggested is a rear face incident type light-receiving device where a light absorption layer (filter layer) having a shorter absorption edge wavelength than that of a light-receiving layer is arranged between a light incident side substrate and the light-receiving layer, such that the filter layer absorbs short wavelength light causing the optical crosstalk, and the light-receiving layer selectively receives only long wavelength light (e.g. Patent Documents 2 to 5). However, so as to sufficiently absorb the short wavelength light causing the optical crosstalk, it is necessary to form the filter layer with a thick film or increase a concentration of impurity. Such countermeasures may degrade crystallinity. Moreover, the filter layer formed of the thick film reduces light to be received, which may lead to low reliability.
According to Patent Document 6, in a rear face incident type light-receiving device, a pn junction is formed in a region other than a light-receiving portion, and light incident from a side face of the light-receiving device is absorbed by a depletion layer formed by the pn junction formed around the light-receiving portion, such that the light is prevented from reaching the light-receiving portion (Refer to Drawing 2 and Paragraphs 0024 and 0041 to 0047). In addition, Drawing 3 of the document shows a construction where a light-receiving portion is formed like a mesa, and a metal light shielding layer is formed on a side face of the mesa, such that the metal light shielding layer shields incident light from the side face of the mesa.
Patent Document 1: Japanese Laid-Open Patent Publication HEI 8-32105 (Drawings 3 and 4)
Patent Document 2: Japanese Laid-Open Patent Publication 2001-28454
Patent Document 3: Japanese Laid-Open Patent Publication 2001-85729
Patent Document 4: Japanese Laid-Open Patent Publication 2003-243693
Patent Document 5: Japanese Laid-Open Patent Publication 2005-101113
Patent Document 6: Japanese Laid-Open Patent Publication 2004-241588 (Drawings 2 and 3)